The surface of a real or computer-generated object can be represented by geometric shapes, such as triangles, that form a piecewise planar approximation of the surface. The geometric shapes collectively form a geometric representation of the surface. Triangles can be defined by three vertices, and a normal, which is a vector that is normal to the plane formed by the triangle. In some examples, a normal can be at a vertex. In some instances, interpolation is used to determine normals that vary gradually across a surface.